Radar level gauges are suitably used for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials. An example of such a radar level gauge can include a microwave unit for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, processing circuitry arranged to communicate with said microwave unit and to determine said level based on a relation between transmitted and received microwaves, an interface for connecting said processing circuitry externally of said radar level gauge, and a power management circuitry providing said microwave unit and said processing circuitry with operating power.
In order to ensure a satisfactory signal level of the received echo, the emitted microwaves must have a sufficient power level. The processing of received signals also requires significant power, and in some cases the clock frequency of the processor is increased during the processing in order to enable high speed calculations. In combination, this results in an increased demand of power during certain parts of the measuring cycle. The power requirements are especially high for Frequency Modulated Continuous Wave (FMCW) systems. However, the provision of power is relatively difficult to achieve in practice, since energy is normally a scarce resource in the above-discussed type of gauges.
In particular, limited available power is a problem in systems using a two wire feeding system. Radar level gauges for measuring of a level in a tank, and other types of process sensors, are typically connected with a two-wire interface, where only two lines serve to both supply the sensor with limited power and to communicate a measured and processed measuring signal. The interface can be a 4-20 mA industrial loop with superimposed digital communication, or another two-wire fieldbus, such as Fieldbus Foundation (FF) or Profibus. Other possible interfaces include a four-wire interface, where two lines provide power, and two wires communicate measurement signals. In case of a 4-20 mA loop, the available power is thus dependent upon the signal value of the gauge, so that during periods with low signal value (e.g. around 4 mA) only a very limited power is available. Even during periods of high signal value (e.g. around 20 mA) the available power may not be sufficient to power the processing circuitry and the microwave emitter during a measurement cycle.
For this reason, power management in some form may be required, to distribute the available power between different components and over time. Such power management may include storage of energy in some kind of energy storage device, so that this energy can be used to boost the available power during periods of increased power requirements. The energy storage can take place in specifically designated stand-by periods, following each measurement cycle, or take place throughout the measurement cycle, during periods of low activity.
In the case where measurements are made in a tank containing explosive gas or liquids, or in any other situation where the sensor is located in an explosion endangered area, there is also an issue of explosion protection. Normally, either the installation is made explosion proof by some kind of encapsulation, or its outside electrical connection is made intrinsically safe (IS). The latter case requires that input power, voltage and current do not exceed levels stated by safety regulations (IS requirements). This is ensured by a so called electrical barrier, arranged in the interface to the intrinsically safe area.
As it is undesirable to capsulate the microwave electronics, the measurement device should operate at a relatively low voltage in order to comply with IS regulations. At such low voltages, energy storage is rendered inefficient as it requires large and slow capacitors. Typically, the minimum energy store capacitance falls within the range covered by aluminium electrolyte capacitors only.
An aluminium electrolyte capacitor looses roughly 20% of its initial capacity due to low temperature (−40 degrees C.) and 20% due to ageing (5000 hours). This may be compensated for at the expense of size, price and a significantly increased start-up time. In addition, if an aluminium electrolyte capacitor is used in a position where a major part of its energy is discharged during the system's active cycle, this will also significantly decrease the initial capacitance value.
Another major disadvantage with using large capacitors is that the life time is reduced dramatically when exposed to high temperatures.
Document U.S. Pat. No. 6,972,584 discloses a power decoupling circuit intended for an ultrasonic level gauge. In order to enable operation of the device, the voltage provided by the current loop is elevated by a step-up converter. Energy storage is also performed at this higher voltage level.